Intracisternal A particles share several biochemical properties with RNA tumor viruses. An RNA methylase has been found in several oncornaviruses. The purpose of this work is to study the RNA methylases in intracisternal A particles. Their presence shows one more similarity with RNA tumor viruses. The fact that cellular methylases are associated with intracisternal A particles suggests a possible role in their life cycle. The intracisternal A particles used were obtained from a MOPC-104E mouse plasma cell tumor maintained by subcutaneous transplantation in Balb/c mice.